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Presented at the NABS Annual meeting, Pittsburgh, Pennsylvania, 2002
in Biogeochemistry II
CONTROLS ON NITRATE SPIRALING IN TWO SMALL FORESTED STREAMS DETERMINED BY 15N ADDITION EXPERIMENTS.
P.J. Mulholland1, S.A. Thomas2, C.A. Ancell3, J.J. Beaulieu1, and A.N. Sullivan1. 1Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6036, 2Department of Biology, Virginia Tech, Blacksburg, VA 24061, 3Department of Biology, University of New Mexico, Albuquerque, NM 87131
To improve our understanding of controls on nitrate uptake in streams, we conducted a series of 15N-nitrate addition experiments in the East and West Forks of Walker Branch, two small, forested streams in eastern Tennessee. Tracer levels of 15N-labelled nitrate were pumped into the stream for 8-12 h and 15N-nitrate was measured in stream water at several locations downstream from the addition. Levels of 15N in all major biomass pools were determined 1 d following the addition to quantify the primary pathways of nitrate uptake. Nitrate uptake length was highly variable over time, reflecting differences in discharge, nitrate concentration, and the demand for nitrogen by stream biota. Nitrate uptake rates per unit area were highest in autumn after leaf-fall and lowest in summer when leaf detritus and primary production were low. Nitrate uptake rates were also relatively high in spring before leaf emergence due to high algal demand, and there was evidence of diurnal variation in nitrate uptake rates at this time of year. Nitrate uptake rates were consistently higher in the East Fork than in the West, and this difference appeared to be the result of uptake within a substantially larger hyporheic zone in the East Fork.
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